The present invention is directed to improved screws adapted to be inserted into bone and a method of making such screws.
The present invention relates to a screw for use in spinal pathologies, deformities and trauma. Because of manufacturing restrictions, the currently available bone screw is not ideal since it is a screw having a tapered core with a constant thread diameter, but with thread apices adjacent to the screw head that are ledged or flattened near the head.
In order to correct spinal conditions spinal bone screws are loaded with axial, distractive and compressive forces and with subsequent cyclically loaded forces applied through the patients natural movement. The bone screws are typically inserted in the vertebral pedicle at a location which has a reduced thickness, or isthmus, midway along its length, and about half way between the posterior bony complex and the thicker vertebral body known as the pedicle.
Previously known bone screws required a relatively large core diameter in order to resist all forces on the screw and to permit the screw to have an uncompromised retention in the pedicle, while at the same time the screw required a minor diameter small enough so that the application will not force a split of the pedicle from the inside causing a condition commonly referred to as xe2x80x9cblow outxe2x80x9d of the pedicle.
Prior art attempts have been made to manufacture a bone screw that has a tapered core with a constant thread diameter while maintaining consistently sharp threads along the entire length of the screw, such as shown in U.S. Pat. No. 5,492,442 issued on Feb. 20, 1996. In practice, the manufacture of the bone screw having a tapered screw core and a constant thread diameter resulted in sharp threads starting from the pointed tip of the screw and advancing along the length of the screw with the last few threads adjacent to the thickest part of the core being ledged or flattened. This produces a situation in which the sharp threads created by the lead thread at the tip of the screw and the threads thereafter are corrupted by the last two or three threads that are ledged or flattened. The result is that the bone becomes microscopically fractured which lessens the pullout resistance of the screw for the bone. The function of the ledged threads on the screw is analogous to a log splitter being used to split a log.
The present invention is directed to improved screws adapted to be inserted into bone and a method of making such screws. The bone screws have a tapered core which is smaller at the leading end and larger at the head. The screw threads have sharp apices, and a constant pitch throughout the length of the screw. The screw has a constant outside diameter throughout the length.
One embodiment of the bone screw has concave superior thread surfaces and ledged threads at the leading end to prevent misalignment during placement. A tap is used in connection with the bone screw attachment.
A second embodiment has flat superior thread surfaces and a constant thread crest throughout the length of the screw. This embodiment may also have ledged threads at the leading end.
The method of making the screw requires process steps including a first tool bit for the initial thread formation and a second tool bit for refining the first set of threads to a constant crest width. The ledged leading threads may be formed at the beginning of the process, between the first and second tool bit, or as an after treatment.
Accordingly, it is an objective of the present invention to provide an improved tapered spinal bone screw having ledged or flattened threads at the beginning of, or lead threads, and in which the following threads have sharp apices, and that the area between threads or thread pitch is constant throughout the length of the threads.
It is another objective of the present invention to provide an improved tapered spinal bone screw having sharp apices along the entire length.
Still another objective of the present invention is the use of a tap in connection with pedicle screw placement. The use of tap alone without altering the bone screw construction could result in serious problems due to the possibility of cross-threading of the screw when introducing the screw into the bone, which consequently alters the path of insertion, that has been established by the tap. If the path is violated or corrupted, serious neurological problems can occur due to the close proximity of the spinal cord. In order to avoid this happening the lead threads of the screw each have a diameter less than the tap core, therefore the tip and some of the following threads of the screw will drop into the hole created by the tap, thus serving as a pilot hole whereby the path of insertion may be maintained, and thus effectively eliminating the possibility of cross threading.
Yet an additional important objective of the present invention to provide an improved spinal bone screw that can be manufactured with present conventional technology.
The screw is provided with a thread configuration, which anchors the screw to the bone and prevents the screw from being axially pulled out from the bone.
Other objectives and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.